The movement of particles into porous materials such as concrete involve several processes, including adsorption, liquid diffusion, capillary absorption, bulk laminar flow, and electrokinetic transport. Electrokinetic transport is the phenomenon of charged particles moving in response to an applied electric field. Electrokinetic transport includes ionic conduction, electrophoresis, and electroosmosis. Ionic solution conductivity accounts for the overwhelming majority of conductivity measured in cement based materials. In an aqueous system (cement concrete structures generally retain a certain moisture content in most conditions), ions can be induced to drift in response to an applied electronic field. Electrophoresis is characterized by the movement of a solid particle dispersed in an electrolyte under the influence of an electric field. Electroosmosis is the induced flow of water through a porous medium such as sand, clay or concrete when an electric potential is applied across the medium.
Depending on the degree of saturation of a concrete sample, any or all of the above transport processes may occur and a number of structural factors may influence the transport processes. Concrete is a mixture of sand, stone (or other aggregate) glued together with a hardened cement paste that is porous. This pore structure is the dominant microstructural feature governing transport. Pore structure originates from the microstructure when water, anhydrous cement grains, and aggregate are mixed. Capillary pore structure initially assumes the shape of the space occupied by mix water. However, hydration of the cement yields calcium silicate hydrate (C—S—H) the primary binder in hardened cement paste. The capillary pore structure is developed as these hydration products form. Capillary pores tend to dominate transport processes and specific structural characteristics of capillary pores which influence transport include pore volume of the sample, size distribution, tortuosity, and connectivity. The aggregate present in the concrete may influence transport in different ways. Low porosity aggregate tends to impeded mass transport by blocking more direct paths through the hardened cement paste pores. Conversely, there can be high porosity at the paste-aggregate interfacial zones. Microcracks and bleed paths also influence particle transport. Microcracks form during drying of the calcium silicate hydrate layers which shrink and lead to tensile stress and cracking. Tensile stress do to plastic shrinkage, stresses from applied loads, thermal expansion or freezing pore water may also inducing microcracking. Bleed paths occur when prior to setting, water accumulates around aggregate and moves toward the surface of the cement paste. Discrete flows can join together to form bleed paths which remain after setting of the cement paste.
Changes in water content of hardened cement pastes have significant impacts on transport mechanisms and rates. At relative humidities above 45%, evaporable pore water is said to exist. Above this threshold, while the permeability of gases is increasingly blocked by liquid water barriers, the transport of aqueous ions or particles progresses more rapidly as the presence of evaporable capillary water increases. Thus, water content is an important factor affecting electrokinetic transport in concrete.